Several applications exist where a circuit that can provide an adjustable impedance at a given frequency (or frequencies) or frequency range (or ranges) is desired or will be advantageous.
One example is transceivers, which are commonly used in a variety of communications devices and comprise a transmitter as well as a receiver.
Such transceivers can be arranged to be operated in semi-duplex, i.e. the receiver and transmitter operate separated in time to prevent the transmitter signal from concealing the received signal. This approach is therefore commonly referred to as time division duplex (TDD).
Transceivers can also be operated in full duplex, i.e. the receiver and transmitter operate simultaneously. In this case, arrangements are provided to prevent the transmitter from concealing the received signal. One approach to achieve this is to assign different frequencies for transmission and reception. This approach is therefore commonly referred to as frequency division duplex (FDD). Often the receiver and the transmitter use the same antenna, or antenna system that may comprise several antennas, which implies that some kind of circuitry may be desired to enable proper interaction with the antenna. This circuitry should be made with certain care when operating the transceiver in full duplex since the transmitter signal, although using FDD, may interfere with the received signal, i.e. internal interference within the transceiver.
In other words, receivers for FDD cellular radio equipment may be subject to very strong signals from their own transmitter. These strong transmit signals are present at duplex distance from the receive channel. The typical duplex distances are small compared to the carrier frequency (typically less than 100 MHz). The receiver must be shielded or isolated from these high power level signals in order to achieve good sensitivity. Currently, this may be achieved by using off-chip acoustic wave duplex filters, called duplexers. A duplexer is arranged to direct radio frequency (RF) signals from the transmitter to the antenna and from the antenna to the receiver. It may e.g. comprise a circulator. Unfortunately duplexers are expensive and bulky, thus increasing the number of components and the required board area. It is also a challenge to implement duplexers on-chip. This is further pronounced by the increasing number of frequency bands to support. Therefore, an integrated solution that performs the isolation function is highly desirable.
Some solutions exist for realizing an on-chip isolation device. One example is disclosed in US 2011/064004, which shows an RF front-end that provides isolation by electrical balance. It is based on cancellation and uses a transformer. For cancellation of transmit signals to occur at the receiver input, symmetry is necessary, and the circuit thus requires a dummy load or balancing impedance, which should be equal to the antenna impedance for perfect cancellation. Although the disclosed balance network is tunable, it does not consider that the antenna impedance is complex (inductive or capacitive) and varies with frequency as well as antenna surroundings. Further, the disclosed balance network is not capable of providing separately tunable impedances at the receive frequency and at the transmit frequency, which is needed since the receive and transmit frequencies are far enough apart for the antenna to present quite different impedances, but still with a frequency distance that is small compared to the carrier frequency.